package offer.middle;

import java.util.Arrays;
import java.util.Scanner;

public class ArithmeticSequence {

    public static String forTrueFalse(int n,int[] array) {
        if (bubble(array)) {
            int i = 0;
            int count = array[1] - array[0];
            while (i < n-1) {
                if (array[i+1] - array[i] != count) {
                    return "Impossible";
                }
                i++;
            }
        } else {
            return "Impossible";
        }
        return "Possible";
    }

    public static boolean bubble(int[] array) {
        boolean flag = true;
        int temp = array[0];
        int count = 0;
        for (int i = 0; i < array.length; i++) {
            if (array[i] == temp) {
                count++;
            }
        }
        if (count == array.length) {
            return true;
        }
        for (int i = 0; i < array.length; i++) {
            for (int j = 0; j < array.length - i - 1; j++) {
                if (array[j] == array[j+1]) {
                    return false;
                } else if (array[j] > array[j+1]) {
                    temp = array[j];
                    array[j] = array[j+1];
                    array[j+1] = temp;
                    flag = false;
                }
            }
            if (flag == true) {
                break;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int length = in.nextInt();
        int[] array = new int[length];
        int i = 0;
        while (in.hasNextInt()) {
            array[i++] = in.nextInt();
        }
        System.out.println(forTrueFalse(length, array));
    }
}
